In recent years, in a semiconductor device, as the speed of an interface (I/F) part is increased and the voltage thereof is reduced, the transistor manufactured in the advanced technology process tends to reduce the voltage that the transistor can handle. However, the majority of interfaces manufactured based on standards not belonging to advanced technology require high voltages. Accordingly, a high voltage output is produced using an output circuit formed by low withstand voltage transistors manufactured in advanced technology process, and in this case, a state where the withstand voltage is not sufficient is brought about.
Consequently, a high voltage output circuit is formed using low withstand voltage transistors by cascode-connecting the low withstand voltage transistors to disperse the voltage applied to the transistors. In such an output circuit, a drive signal the level of which is shifted is applied to the gates of a part of the transistors and at the same time, a bias voltage is applied to the gates of the other transistors.
In the output circuit, the drive signal and noise from the output node affect the bias voltage and fluctuate the bias voltage. If the bias voltage fluctuates, there is a case where the voltage applied to the transistor exceeds the withstand voltage, and therefore, the transistor is destroyed.
In the case where a high frequency operation is performed in such an output circuit, the device size of PMOS transistors and NMOS transistors that appears when viewed from the output terminal is increased. Accordingly, the capacitance between the gate and drain of the transistor also increases and the AC fluctuation component at the output terminal largely affects the node of the bias voltage via the capacitance. Because of this, the bias voltage fluctuates and if the fluctuations are large, it is no longer possible to guarantee the withstand voltage.
Consequently, a bypass capacitor is connected between the signal line of the bias voltage and a reference voltage source (GND) and thereby the fluctuations in the bias voltage due to noise are suppressed. However, in general, the capacitor used within LSI increases the size of the LSI, and therefore, if a large-sized capacity is provided, the size of LSI is increased and if the size of the capacitor is reduced, the capacitance is reduced, and therefore it is not possible to sufficiently reduce noise.